1. Field of Invention
The field of the currently claimed embodiments of this invention relates to surgical instruments and systems that incorporate the surgical instruments, and more particularly to systems and surgical instruments that have integrated force sensors.
2. Discussion of Related Art
In current practice, retinal surgery is performed under an operating microscope with free-hand instrumentation. Human limitations include an inability to clearly view surgical targets, physiological hand tremor, and lack of tactile feedback in tool-to-tissue interactions. In addition, tool limitations, such as lack of proximity sensing or smart functions, are important factors that contribute to surgical risk and reduce the likelihood of achieving surgical goals. Current instruments do not provide physiological or even basic interpretive information, e.g. the force exerted by the instrument on the retinal tissues. Surgical outcomes (both success and failure) are limited, in part, by technical hurdles that cannot be overcome by conventional instrumentation.
The tool-to-tissue interaction forces commonly encountered in retinal microsurgery are generally far below human perceptual limits [1-4]. If too much force is exerted on the retina, then it may be damaged. Typically, surgeons rely entirely on visual appreciation of tissue deformation to estimate how close tool-to-tissue forces are approaching to unacceptable limits. However, this skill is not easily learned and making such estimates is difficult even for an extremely skilled surgeon. Similar challenges may be found in other microsurgical disciplines, including non-retinal ophthalmic surgery, neurosurgery, otologic surgery [6-9], micro-vascular surgery, etc. Even in surgical situations where the actual tool-to-tissue forces may be larger than encountered in microsurgery, such as endoscopic surgery, the tool-to-tissue forces may be difficult to measure or for the surgeon to appreciate directly, due to friction, mechanical constraints, manipulation limitations, etc. These considerations have led a number of researchers to consider methods for incorporating force sensors into surgical instruments. This force information may be used in various ways to assist the surgeon, including use in “sensory substitution” (e.g., [1, 3, 10-12]), in various forms of haptic feedback (e.g., [10, 15]), or otherwise incorporated into control of robotic devices (e.g., [10, 16]).
For example, in retinal surgery (FIG. 1) the surgical tool is inserted through the sclera into the eye to perform the manipulations of delicate tissue during the retinal surgery. To measure the forces exerted between the tissue and the tool tip is very challenging. The force sensing instrument should be capable of measuring the tissue-to-tool force in three dimensions with sub-millinewton resolution. The sensor should be placed possibly close to the tool tip inside of the eye to avoid disturbance of sclera-to-tool forces. Thus the sensor must possess the essential small size so that it can be integrated into the instrument with sub-millimeter diameter. Further requirements include biocompatibility, sterilizability and immunity to electrical noise.
One method for measuring tool-tissue forces is incorporation of a (typically, multiple degree-of-freedom) force sensor into the handle of a surgical tool. This method has been applied for microsurgical force sensing experiments (e.g., [4, 9]). However, in cases such as retinal surgery, where tool-to-sclera interaction forces can be as large as or larger than tool-to-tissue interaction forces, this approach has serious drawbacks. Consequently, there has been interest in developing microsurgical force sensors placed on the distal portion of the surgical tool, below the insertion through the sclera [13, 14]. Although some similar approaches (e.g., [17-19]) have been undertaken for laparoscopic tools, where tool-to-trocar forces can similarly mask tool-to-tissue forces, the challenge for micro-retinal surgery, where the tool shaft diameters can be 0.5-0.7 mm or even smaller, is especially severe. Some early work at Johns Hopkins [1] used a strain gauge mounted on a tool shaft to measure 1 degree-of-freedom (DOF) forces deflecting the tool in “open” experiments on dissected pig retinas, but this tool was not practical for insertion through the sclera. Further approaches include incorporation of electrical sensors (such as strain gauges) into tiny microsurgical instruments.
There have been several optical fiber force sensors proposed for surgical applications (e.g., [13, 14, 20-23]), as well as several other optical approaches (e.g., [24, 25]) for surgical force sensing. These sensors use a variety of physical principles, including modifications in reflected or transmitted light intensity, changes in polarization, fiber-Bragg grating (FBG) sensing, etc. FBG sensors are constructed by producing a modulation of the index of refraction (i.e., a “grating”) along the length of an optical fiber [20]. Stretching the fiber introduces a change in the spacing of this grating and, hence, of the wavelength of light reflected back up the fiber. This wavelength shift is measured to determine the amount of strain in the grating portion of the fiber. In addition to our work [13, 14], others have applied FBG force sensors to other sorts of surgical instrumentation. For example Mueller et al. [22] have reported a 6-DOF FBG-based force/torque sensor based on FBG sensors that would be suitable for mounting in the tool handle or proximal end of robotic surgical instruments, in a manner somewhat analogous to [9].
Optical fiber-based sensors have many advantages for microsurgical applications. The fibers and sensors are inexpensive. They are sterilizable by a variety of common means. They can be made biocompatible. They can be made very small. They are immune to electrical noise and magnetic fields. They involve no electrical currents. A number of sensing principles may be used to measure delicate displacements and forces. Further, although this is not a consideration for retinal surgery, they are MRI compatible.
In previously published work [13, 14], our JHU team has incorporated FBG fibers into the tool shafts of 0.5-0.7 mm microsurgical instruments to make 1-DOF and 2-DOF force sensing tools with force resolutions on the order of 0.25 mN. Our team has also developed 3-DOF FBG based microsurgical instruments. One concept for a 3-DOF force sensing tool is shown in FIG. 2A. Here, lateral forces are measured by FBG sensors; just as in our 2-DOF tools, and axial forces are measured by a force sensor in the handle. The advantage of this scheme is simplicity. However, one major drawback is that, although ambiguities introduced by lateral sclera-tool interactions are eliminated, axial ambiguities introduced by sclera-to-tool friction are not.
This consideration has led us to develop prototype 3-DOF tools with all sensing inside the eye [26], as shown in FIG. 2B. In this design, FBG sensors along the sides of the shaft measure lateral deflections of the tool shaft as a result of lateral forces, much as is the case with our 2-DOF tools. In principle, the axial extension of the tool shaft in response to axial forces could also be measured, but there are several drawbacks. First, the tool is very stiff in the axial direction, resulting in low sensitivity. Second, thermal expansion of the tool can also stretch all the fibers. Consequently, lateral forces are actually computed differentially, using 3 FBG sensors to compute two lateral forces. For axial forces, we use a fourth FBG fiber running axially through the middle of the tool and attached to the distal end of a micro-machined compliant section of the tool shaft.
There are, however, a number of problems with microsurgical sensors based on FBG sensing, which our current invention seeks to address. Some of these problems include:                The FBG fibers must be firmly attached along the sides of the tool. This can involve a difficult manufacturing process, and glue compliance, viscous response, and hysteresis between the FBG and the tool shaft can introduce problems. There are similar (and even more challenging) manufacturing and attachment problems associated with the “axial” fiber, which must also be pre-loaded.        The FBG fibers have significant stiffness. This especially affects the axial force sensitivity of the tool, since the mechanical advantage from the bending of the tool shaft on the stretching of the lateral FBG fibers is lost.        The micro-machined grooves into the tool shaft to produce a compliant section, as shown in FIG. 2B, can become clogged with material, thus affecting the tool's calibration and also creating cleaning/sterilization problems if the tool is to be reused.        Obtaining good signal-to-noise characteristics requires that the FBG grating be fairly long (typically about 10 mm on our microsurgical tools). This constrains the design of the tool and also can cause problems if external forces are exerted on that portion of the tool shaft containing or proximal to the grating.        
There thus remains a need for improved surgical tools and systems for microsurgical applications.